Using the Necromancer, an elemental snowstorm (the one with the giant snowball) hit at the same time I cast Leech against skeletons in the area next to the door from Cathedral 4 to Royal Crypts. The whole screen turned the red color of the Leech spell, then the Orange of the door, then there were white splotches like the snow effect, but blinking, and then the whole screen turned grey as the snowball bounced around faintly before disappearing into the grey background. After about 20 seconds the effects faded, the Leech spell and snowstorm ended, and the visuals went back to normal. I’ve reloaded and gone back to the same spot a few times, to recreate the effect, but it hasn’t happened again.